Switches, such as field-effect transistor (“FET”) or micro-electro-mechanical system (“MEMS”) switches, are used extensively in analog, mixed-signal, and radio-frequency (“RF”) applications. RF applications may use switches for signal routing (e.g., as transmit/receive switches), as switched attenuators, switched resonators, switched capacitors, or as capacitor band-switched voltage-controlled oscillators (“VCO”). A switch may also be used as, for example, a mixer.
One example of a FET switch is an n-type metal-oxide-semiconductor (“NMOS”) transistor pass-gate, wherein the input signal is received at the transistor's source, the output is generated by the transistor's drain, and the gate and back gate of the transistor control the state of the switch.
This FET switch may exhibit poor performance at high frequencies, such as RF frequencies. The NMOS transistor may be sized such that its on-resistance (“Ron”) provides a low-resistance signal path from the switch's input to its output. At this size, however, the transistor may have large parasitic capacitances connecting the source and drain to the transistor's gate and back gate. At high frequencies, the gate and back gate may have low impedances, and the parasitic capacitors may short the switch's input signal to these low impedance nodes and thereby limit the bandwidth of the switch. Furthermore, a varying signal applied to the transistor's input may modulate the conductance (Gds) of the transistor, which may produce intermodulation distortion (“IMD”) and reduce the linearity of the switch. A similar effect may be observed with any switch that capacitively links an input signal to a control node, such as a MEMS switch.
Resistors may be added in series with the control node(s) to mitigate these problems. The resistors may limit the effects of the parasitic capacitances by raising the impedances of the control node(s), which may prevent the input signal from shorting to the control node(s), thereby reducing signal loss and improving the bandwidth of the switch. In addition, by raising the impedance of the control node(s), the resistors may “free” the control node(s) to track changes in the input voltage applied to the transistor's source—a method of self-bootstrapping. For example, as the FET switch's gate-source voltage (“Vgs”) changes in response the input voltage, the resultant reduction in Gds variation may reduce IMD.
These resistors, however, provide a means for the switch's input to couple capacitively to the switch's output, even when the switch is in an off-state. Thus, while the resistors may reduce the on-state loss of the switch, they also undesirably affect the off-state isolation of the switch. A need exists to obtain the benefits of series-connected control node resistors without their drawbacks.